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Date:   Sun, 14 Aug 2022 19:14:31 +0200
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Subject: Re: [PATCH] drm/amd/amdgpu: Replace kmap() with kmap_local_page()
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To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Nirmoy Das <nirmoy.das@amd.com>,
        Jonathan Kim <jonathan.kim@amd.com>,
        Philip Yang <Philip.Yang@amd.com>,
        Matthew Auld <matthew.auld@intel.com>,
        Zack Rusin <zackr@vmware.com>, amd-gfx@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org
Cc:     Ira Weiny <ira.weiny@intel.com>
References: <20220812175753.22926-1-fmdefrancesco@gmail.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
In-Reply-To: <20220812175753.22926-1-fmdefrancesco@gmail.com>
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Am 12.08.22 um 19:57 schrieb Fabio M. De Francesco:
> kmap() is being deprecated in favor of kmap_local_page().
>
> There are two main problems with kmap(): (1) It comes with an overhead as
> mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
>
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and are still valid.
>
> Since its use in amdgpu/amdgpu_ttm.c is safe, it should be preferred.
>
> Therefore, replace kmap() with kmap_local_page() in amdgpu/amdgpu_ttm.c.
>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

For those two use cases that looks good to me. Feel free to add an 
Acked-by: Christian König <christian.koenig@amd.com>

Regards,
Christian.

> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> index 3b4c19412625..c11657b5915f 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
> @@ -2301,9 +2301,9 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
>   		if (p->mapping != adev->mman.bdev.dev_mapping)
>   			return -EPERM;
>   
> -		ptr = kmap(p);
> +		ptr = kmap_local_page(p);
>   		r = copy_to_user(buf, ptr + off, bytes);
> -		kunmap(p);
> +		kunmap_local(ptr);
>   		if (r)
>   			return -EFAULT;
>   
> @@ -2352,9 +2352,9 @@ static ssize_t amdgpu_iomem_write(struct file *f, const char __user *buf,
>   		if (p->mapping != adev->mman.bdev.dev_mapping)
>   			return -EPERM;
>   
> -		ptr = kmap(p);
> +		ptr = kmap_local_page(p);
>   		r = copy_from_user(ptr + off, buf, bytes);
> -		kunmap(p);
> +		kunmap_local(ptr);
>   		if (r)
>   			return -EFAULT;
>